1. Field of the Invention
The present invention relates to a magnetic field generator and a method of driving the same and, more particularly, to a magnetic field generator suitably used as a biasing magnetic field generating means of an optomagnetic data (information) recorder and capable of switching the direction of a magnetic field at a high speed and to a method of driving the magnetic field generator.
2. Description of the Prior Art
An electromagnetic converter using an electromagnet is conventionally known as a device for generating a magnetic field.
Such a magnetic field generating means is used as a magnetic head of a tape recorder and the like. In this case, since the magnetic head and a tape are brought into contact with each other to record/reproduce data, a high magnetic flux density need only to be obtained near the magnetic head. Therefore, the device can be made small in size, and a good response of the electromagnet can be obtained. In addition, since magnetic fluxes can be concentrated by adjusting a gap and the like between both right and left poles of the magnetic head, a high magnetic flux density can be obtained even if the device is made small in size.
On the other hand, in an optomagnetic recorder which has been recently noted as a large capacity data memory, the above magnetic field generator is used to obtain a biasing magnetic field during recording or erasing of data. For example, U.S. Pat. No. 4,597,018 describes an optomagnetic recorder including a magnetic field generating means. In addition, U.S. application Ser. No. 866,314 filed May 23, 1986 assigned to the same assignee describes a process of data recording by applying a biasing magnetic field. Furthermore, U.S. application Ser. No. 620,808 filed June 14, 1984 (now U.S. Pat. No. 4,686,661), Ser. No. 904,453 filed Sept. 8, 1986, and Ser. No. 907,749 filed Sept. 16, 1986, all assigned to the same assignee as the present invention, describe an example of a magnetic field generator using an electromagnet.
In such a magnetic field generator used in an optomagnetic data recorder, a magnetic flux density having a magnetic field of several hundreds to several kOe is required even at a position apart from an electromagnet by several mm. This is because it is difficult to move a magnetizing medium within a range of about 2 mm to the electromagnet since the magnetizing medium is located inside of the magnetic disk surface by about 1.2 mm and when the disk is rotated, the disk surface is vertically moved by about 1 mm due to vibration.
Therefore, in the optomagnetic data recorder, a relatively large electromagnet must be used to obtain a predetermined magnetic flux density.
In addition, in the optomagnetic data recorder, the direction of a biasing magnetic field must be switched between data recording/reproducing operations. If this recording/reproducing switching operation is not performed rapidly, a data transfer speed is undesirably decreased. For this reason, according to a conventional driving method, a high voltage is applied across a coil of an electromagnet to cause a magnetic field to rise, and then the voltage is decreased to hold a constant magnetic field.
In such an electromagnet, in order to switch a magnetic field at a high speed, a coil having a large diameter wire with a small number of turns must be used, and a large current must be flowed therethrough. On the other hand, a large ampere-turn is necessary to hold a large magnetic flux. When a small current is flowed to suppress heat generation, the number of turns of the coil must be increased. However, according to the above-mentioned electromagnet, the magnetic field is switched and held by a single coil. Therefore, when the number of turns of the coil is increased, a high voltage power source is required, thereby increasing a power consumption at a driving stage. On the contrary, when the number of turns of the coil is decreased, a power consumption of the electromagnet is increased, thereby increasing the amount of generated heat.